1. Field of the Invention
This invention relates to a cutting tool comprising a supporting base and an outside and an inside abrasive grain layer projecting from the supporting base concentrically in a radially spaced relationship, and to a method of its production.
2. Description of the Prior Art
Optical or magnetic discs have found wide commercial acceptance as an optical or magnetic recording medium. The base plate of such an optical or magnetic disc has previously been formed of a metallic material such as aluminum, but in recent years, it has been proposed to make optical or magnetic discs from glass or ceramics for various reasons including the need to increase surface flatness and smoothness.
In the production of the base plate from glass or ceramics, it is generally necessary to cut a square or rectangular glass or ceramic plate along concentric circular outside and inside cutting lines and to obtain a disc-shaped base plate remaining between the circular outside and inside cutting lines. It is important to perform the cutting along the circular outside and inside cutting lines very precisely, for example by limiting the circularity of each of the inner circumferential edge and the outer circumferential edge of the base plate and the concentricity between the two to less than several tens of micrometers to several micrometers without forming excessively large chippings, for example those having a dimension of at least 0.2 mm in an arbitrary direction, on the inner circumferential edge and/or the outer circumferential edge of the base plate.
As is well known, however, glass or ceramics have high hardness and brittleness, and are much more difficult to cut than metal. Hence, no cutting tool has been developed which can be applied to the production of a base plate for optical or magnetic discs by efficiently cutting a glass or ceramic plate.
A cutting tool of the structure disclosed in the specification of Japanese Laid-Open Utility Model Publication No. 136610/1982 may be used for cutting a glass or ceramic plate as above. This cutting tool has a cylindrical outside support and a cylindrical inside support disposed concentrically, and a plurality of arcuate grinding stone (i.e., cutting segments) formed by bonding abrasive grains with a suitable binder are bonded by silver solder to the front end of each of the outside and inside supports in a circumferentially spaced relationship. In the production of this cutting tool, it is difficult to form the cutting segments themselves with sufficient precision. It is furthermore impossible or extremely difficult to bond the cutting segments sufficiently precisely to the required positions of the front end of each of the outside and inside supports. It cannot at all be expected, therefore, that this cutting tool will meet the aforesaid requirements.